8 th Jun SEM X International Congress X-Ray Microdiffraction on Diamond-shaped NiTi for Biomedical Applications Apurva Mehta SSRL/ SLAC, Stanford University Valentina Imbeni

Apurva Mehta 8 th Jun. 2004SEM X International Congress New Boss

Apurva Mehta 8 th Jun. 2004SEM X International Congress Collaborators  Valentina Imbeni – SRI  Brad Boyce – Sandia Labs  Nobumichi Tamura – LBL  Xiao-Yan Gong, Alan Pelton, & Tom Duerig – NDC  Rob Ritchie’s Group (Scott Robertson, Monica Barney) – LBL/ UC Berkeley

Apurva Mehta 8 th Jun. 2004SEM X International Congress Motivation: Macroscopic  ---  Microscopic  Understanding of Deformation and Failure of NiTi components at Local Level under Multiaxial Loading.  Validation of Design Models.  Towards Improved Models that include:  Austenite to Martensitic Phase Transition  Mechanics Beyond Continuum Mechanics. In vivo loading fractures

Apurva Mehta 8 th Jun. 2004SEM X International Congress Motivation E.g., understanding Fatigue Tests  Location of Fracture  Increase of Fatigue Life Above 1.5% Strain !! A. Pelton et. al. - NDC

Apurva Mehta 8 th Jun. 2004SEM X International Congress Talk Outline  What did we do?  Methodology  What did we find?  Diamond in Compression  Diamond in Compression Cycling  Diamond in Tension  Five “New” Insights

Apurva Mehta 8 th Jun. 2004SEM X International Congress Methodology Load Cell FEA Simulations X-ray Beam Tension Nitinol Tube 4.67mm OD with 0.38mm wall Laser machined Fully Annealed – Grains ~ microns compression

Apurva Mehta 8 th Jun. 2004SEM X International Congress Bend Magnet Source (250x40  m) 1:1 Toroidal mirror 1:1 image at slits Elevation view Plan view 4 Crystal Si(111) Monochromator CCD camera Sample on scanning XY stage Horizontal focusing K-B mirror Vertical focusing K-B mirror Methodology X-ray Microdiffraction Beam size on sample: 0.8x0.8  m 2 Photon energy range: 5-14 keV Schematic layout of the X-ray Microdiffraction Beamline (7.3.3.) at the ALS

Apurva Mehta 8 th Jun. 2004SEM X International Congress Methodology X-ray Microdiffraction-1 micron spot NiTi Diffraction Patterns 10  m Grain Map Elastic Strain Plastic Strain Ni & Ti Fluorescence Austenite Diff. Pattern

Apurva Mehta 8 th Jun. 2004SEM X International Congress DeviatioricDilational From energy scan (Variably Monochromated X-rays) From Laue Patterns deviations (broad bandpass (White) X-rays) Strain Tensor Strain Tensors In crystal reference frame  xx  xy  xz  xy  yy  yz  xz  yz  zz + Crystal Orientation From Laue Patterns In Sample reference frame

Apurva Mehta 8 th Jun. 2004SEM X International Congress Displacement  Strain

8 th Jun SEM X International Congress Findings

Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 0 mm : F = 0 N  xx  yy

Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 0.5 mm : F = N  xx  yy

Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 1.0 mm : F = N  xx  yy

Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 1.5 mm : F = N  xx  yy

Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 2.5 mm : F = N  xx  yy

Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 3.7 mm : F = N  xx  yy

Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 3.7 mm : F = N Phase Map  yy Austenite Martensite

Apurva Mehta 8 th Jun. 2004SEM X International Congress Insight #1 Finite Elem. Analysis Microdiffraction 3.7 mm compression Qualitative agreement with FEA But – Granular and Speckled X. –Y. Gong et al.

Apurva Mehta 8 th Jun. 2004SEM X International Congress Insight #2  Local Strain Never exceeds 1.5 %  NiTi Superelastic because the Aust. And Mart. Elastic region separated by a large region of Transformation Strain Martensite Austenite  Molar vol ~ strain 1.5% Aust + Mart 2 phase region Transformation const. Stress

Apurva Mehta 8 th Jun. 2004SEM X International Congress Insight #3  Strain relief on transformation  Strain reversal Austenite Molar vol ~ strain 1.5% Nucleation energy

Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 2.5 mm unload : F = N  xx  yy

Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 0.0 mm unload : F = N  xx  yy

Apurva Mehta 8 th Jun. 2004SEM X International Congress Load mm Zero Cycles 0 – 3.7 mm One Cycles mm Eleven Cycles 4.9 – mm

Apurva Mehta 8 th Jun. 2004SEM X International Congress Insight #4  On cycling Martensitic region grows.  Growth Pattern unpredictable from FEA  Strain relief as Martensite grows  Explanation for increased Fatigue Life for macroscopic strains > 1.5 %

Apurva Mehta 8 th Jun. 2004SEM X International Congress Tension :  yy

Apurva Mehta 8 th Jun. 2004SEM X International Congress Insight #5  Transformation front and hence stress “hotspot” changes direction, and traverses down the stem of the diamond.  Failure occurs when the “hotspot” encounters a defect or weakness in the material. Location of failure maybe different from FEA prediction.

Apurva Mehta 8 th Jun. 2004SEM X International Congress Summary  Insights:  Strain map granular, martensite evolution speckled.  In the superelstic region max stress doesn’t exceed stress corresponding to 1.5% Austenite strain.  Strain relief and strain reversal at the transformation front.  On load cycling, the martensite region grows. Overall stress drops.  Transformation and max stress front changes directions.  Further Questions:  What is the crystallographic relationship between the Martenite and the Austenite phase?  What happens around a crack tip?

Apurva Mehta 8 th Jun. 2004SEM X International Congress Crystallographic Relationships

8 th Jun SEM X International Congress Thanks !